Solar cells are power generating systems which do not discharge carbon dioxide during power generation and have little adverse effect on the environment, and in recent years, solar cells have been rapidly popularized.
A solar cell module in general has a structure in which a solar cell is sandwiched between a front surface glass on a side where sunlight enters, and a so-called back sheet that is disposed on a side opposite to a side where sunlight enters (rear surface side). The spaces between the front surface glass and the solar cell and between the solar cell and the back sheet are respectively sealed with an EVA (ethylene-vinyl acetate) resin or the like.
The back sheet has a function of preventing the penetration of moisture from the rear surface of a solar cell module, and glass, a fluororesin and the like have been traditionally used. However, in recent years, polyester has been increasingly used from the viewpoint of cost. Furthermore, the back sheet is not necessarily a mere polymer sheet, but may be imparted with various functions such as those described below.
Regarding the functions described above, for example, there may be a demand for imparting a reflection performance by adding white inorganic fine particles such as titanium oxide to the back sheet. This is because when the portion of light in the sunlight that is incident through the front surface of the module and passes through the cell is diffusely reflected and returned to the cell, the power generation efficiency is increased. In this regard, an example of a white polyethylene terephthalate film with added white inorganic fine particles has been disclosed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2003-060218 and JP-A No. 2006-210557), and an example of a rear surface protective sheet having a white ink layer containing a white pigment has also been disclosed (see, for example, JP-A No. 2006-210557).
Furthermore, there are cases where the back sheet is required to have decorativeness. In this regard, there has been disclosed an example of a back sheet for a solar cell to which a perylene pigment, which is a black pigment, has been added to improve decorativeness (see, for example, JP-A No. 2007-128943).
Furthermore, there are cases where a polymer layer is provided as the outermost layer of a back sheet in order to obtain strong adhesion between the back sheet and an EVA sealing material. In this regard, there has been disclosed a technology of providing a thermally adhesive layer on a white polyethylene terephthalate film (see, for example, JP-A No. 2003-060218).
In order to impart functions such as described above, the back sheet has a structure in which a layer having another function is laminated on a support. Examples of lamination methods may include a method of affixing sheets having various functions on a support. For example, there has been disclosed a method of forming a back sheet by affixing plural resin films (see, for example, JP-A No. 2002-100788). Furthermore, as a method of forming a back sheet at lower cost than the method of affixing, there has been disclosed a method of coating layers having various functions on a support (see, for example, JP-A No. 2006-210557 and JP-A No. 2007-128943).
Further, there is a disclosure regarding a white polyester film for a reflective plate in which a coating layer containing an antistatic agent and a silicone compound is provided on a white polyester film, or a back sheet for a solar cell in which an adhesion layer containing an epoxy resin, a phenol resin, a vinyl copolymer or a siloxane compound is laminated on an organic film (see, for example, JP-A No. 2008-189828 and JP-A No. 2008-282873).
However, although there are available technologies disclosed in connection with the method of forming a back sheet by affixing, these technologies involve high cost, are inferior in interlayer adhesiveness in long-term use, and are unsatisfactory in terms of durability. That is, since back sheets are directly exposed to moisture, heat or light, the back sheets are required to have durability with respect thereto for a long time. For example, back sheets generally have a structure adhered to an EVA sealing material, and in this case, the adhesion durability over time between the back sheet and the EVA is extremely important. Furthermore, the adhesion durability between the support and the respective layers is also indispensable.
There have been also disclosed methods involving coating. However, it is difficult to maintain adhesiveness for a long time in an environment with relatively high temperature and humidity, and these methods are not yet satisfactory in providing a back sheet for a solar cell which can be produced at low cost and achieves a good balance between light reflectivity or the like and adhesiveness to an EVA sealing material.
Regarding the above polyester film or back sheet containing a silicone compound or a siloxane compound, the former is inferior in the durability of the cationic polymer contained as an antistatic agent, and the latter is inferior in the durability of a resin or a copolymer other than the siloxane compound, so that it is difficult to maintain the adhesiveness over a long period of time in an environment with relatively high humidity and temperature.
As described above, in the current situation, there has not yet been provided a back sheet for a solar cell, which has both adhesiveness to EVA sealing materials that lasts for a long time and other functions (for example, reflection performance or decorativeness), and which, at the same time, can be produced at low cost and can provide satisfactory durability with respect to moisture and heat.